1. Field of the Invention
The present invention relates generally to the rolling of metal ingots and, more particularly, to methods and apparatus for increasing ingot rolling yields and rolling mill efficiency by minimizing end crop losses in the rolling of flat slabs, for example. This favorable increase in material yield and rolling efficiency is achieved by a novel slab ingot end geometry and formed in one or both ends of the ingot, preferably during ingot casting. The invention is most advantageously applied to the manufacture of aluminum mill products.
2. Description of the Prior Art
A widely used method of manufacturing aluminum plate, sheet and foil products initially involves the vertical semicontinuous casting of slab-shaped ingots which includes a bottommost leading end, referred to in the art as the xe2x80x9cbuttxe2x80x9d of the ingot. The butt is formed as the liquid metal solidifies on the movable bottom block or starter block which is in the open bottom of the mold. After solidification, the butt assumes the shaped geometry of the bottom block. The bottom block continuously moves downwardly and away from the mold as the solidified metal ingot exits at the open end of the mold at the location previously occupied by the bottom block. The cross-section of the vertically cast ingot of metal assumes the horizontal cross-sectional geometry of the mold. The sidewalls of the mold and the sidewalls of the solidified ingot exiting the mold are sprayed with water to increase the solidification rate. This casting technique is referred to as direct chill or xe2x80x9cDCxe2x80x9d casting, all of which are well-known in the art. After the cast ingot has reached a desired length, the molten metal flow to the mold is terminated and the solidified ingot is removed from the casting pit for further processing. It is common practice in commercial DC casting to pour a plurality of ingots in a casting run from a plurality of side-by-side molds. Of course, it will be readily understood by those skilled in the art that the present invention is suitable for use in conjunction with other semicontinuous casting systems such as, for example, electromagnetic casting (EMC casting).
The DC or EMC cast ingots may then be scalped to remove as-cast surface imperfections and homogenized by heating in a furnace to provide a uniform chemistry across the ingot cross-section prior to rolling. In order to process the thus treated ingots to useful end products, such as sheet, plate, foil or the like, the ingots are heated to a desired rolling temperature and subjected to a plurality of hot rolling roughing passes in a slabbing mill. Such rolling mills conventionally use one or more reversing roughing mill stands.
The free surfaces existing on an ingot or slab of finite width, thickness and length allow nonuniform rolling deformation to occur in the length and width dimensions during hot rolling. This nonuniform deformation causes an elongation of the slab in the center region thereof which forms a convex, longitudinally extending xe2x80x9ctonguexe2x80x9d condition at the ends thereof, particularly in aluminum slabs which are roughed down in reversing mills, usually without the use of side or edge rolls. Formation of a tongue condition is, however, not uncommon in the rolling of aluminum even in mills equipped with edge rolls. The aforesaid nonuniform deformation phenomenon is more severe in the length direction of the slab leading to another condition referred to in the art as xe2x80x9cfold overxe2x80x9d, xe2x80x9coverlapxe2x80x9d or xe2x80x9calligatoringxe2x80x9d. These objectionable conditions at the ends of the slab grow worse as rolling continues and must eventually be removed by a crop shear to permit further rolling to continue. Some mills have a limitation on the crop length, due to crop shear equipment limitations, and must take two or more crops to crop off the necessary length dictated by the overlap and tongue deformations. In some cases, it has been observed that severe slab end elongation may occur during the early rolling passes which would ideally call for removal by intermediate end cropping but may not be possible if the slab thickness is too great for the crop shear. In such cases, the end deformation then worsens, causing additional end crop losses as rolling continues. It is known that less cropping length provides obvious metal recovery benefits and/or operational benefits if cropping can be postponed to later rolling operations. In addition, it is known that the overlap or alligatoring phenomenon may sometimes, in severe cases, cause the upper and lower surfaces of the slab to flair upwardly and downwardly beyond the ends of the slab at the horizontal centerline. This overlap must be sheared to allow rolling to lower thicknesses for safe entry into continuous mill equipment. In addition, the flared ends of the xe2x80x9calligatorxe2x80x9d move or otherwise damage table roll surfaces and work rolls which disrupts production. It is also well-known that the overlap causes an internal lamination crevice in the metal which grows during rolling and will result in unsound plate and sheet products unless it is removed by crop shearing.
Previous experimental work has been undertaken by Applicants"" colleagues in an effort to reduce slab rolling cropping losses by tapering the ends of slab ingots by machining away the upper and lower transverse edges of the ingot so as to form a somewhat truncated, arrow-shaped end profile when the ingot is viewed in a longitudinal side view. In-house tests were run on ingots having 30xc2x0, 38xc2x0 and 45xc2x0 tapered ends. The optimum shape was noted to be between a 30xc2x0 and 34xc2x0 taper to reduce the xe2x80x9cfoldoverxe2x80x9d, xe2x80x9coverlapxe2x80x9d, xe2x80x9calligatoringxe2x80x9d problem. This 30xc2x0-34xc2x0 deep taper achieved by machining represents an added cost to the manufacturing process and also causes some material loss. In addition, while the xe2x80x9calligatoringxe2x80x9d problem was reduced somewhat by the machined tapered ends, the xe2x80x9ctonguexe2x80x9d elongation problem, i.e., a convexly shaped protruding end (in plan view) was still present.
A process for preventing the growth of xe2x80x9cfish mouthxe2x80x9d overlap is proposed in U.S. Pat. No. 4,344,309 to Matsuzaki dealing with the rolling of steel slabs. Recesses are formed at the ends of the steel slab by partially rolling the ends of the slab in several short reverse rolling bites which are said to minimize the formation of overlaps in steel slabs. Recesses are also formed in the widthwise direction at opposite side edges of the ends of the slab by vertically extending side rolls in the same manner in an attempt to prevent the formation of fishtails. Rolling then progresses to reduce the steel slab, with additional side edge rolling, with the formation of intermediate recesses required. This elaborate rolling schedule which is said to minimize the formation of overlaps and fishtails in steel slabbing requires additional rolling time and, thus, adds cost to the end product. In addition, many slab roughing mills, particularly in the aluminum industry, are not equipped with the vertical side rolls required in U.S. Pat. No. 4,344,309. The literature also suggests the shaping of steel ingot ends by forming a truncated pyramid shape at the bottom end of an ingot to minimize cropping losses while employing edge rolling of steel slabs. Once again, these proposals are not applicable to aluminum roughing mills which do not employ side or edge rolling using vertically oriented rolls.
The present invention overcomes the shortcomings of the prior art by providing a method, apparatus and shaped slab ingot for reducing hot mill end crop on at least the butt end of a slab which greatly improves mill productivity and metal yield, particularly in the hot rolling of aluminum mill products.
The present invention contemplates a method, a product and apparatus which provide an ingot having a special configuration formed on at least the butt end of an ingot, preferably formed during casting thereof. The specially configured slab ingot provided by the present invention minimizes the occurrence of overlapping/alligatoring as well as tonguing during slab rolling, thus reducing the cropping losses to increase mill productivity and metal recovery.
The invention further provides a specially shaped bottom block used in the slab ingot casting to provide a shaped butt end in the ingot to minimize overlapping/alligatoring and tongue formation during subsequent hot rolling/slabbing. Controlling the end shape of the ingot in accordance with the present invention provides easier cropping due to the fact that the rolled ingot is thinner at the time when cropping is required. Still further, the present invention contemplates the use of a xe2x80x9chot topxe2x80x9d type mold to place on the mold at the conclusion of the ingot casting pour to form a special shape at the head end of the ingot similar to the shape at the butt end. Hence, the common rolling crop loss problems relating to tongue and overlap/alligatoring are minimized at the head end as well. Still further, the present invention provides a slab ingot having at least one end specially shaped by casting or by machining to reduce the formation of tongue and overlap problems during rolling.
The present invention contemplates an apparatus comprising a specially shaped bottom block or starter block for imparting a like shape to the butt end of a direct chill (DC) or electromagnetic cast (EMC) cast aluminum slab ingot. The invention also is directed to a slab ingot having one or both of its ends specially shaped, either by molding and/or by machining. The invention further includes a process or method for reducing end crop losses in the rolling of metal slab ingots by providing a slab ingot having at least one specially shaped end by molding and/or machining the special shape. The invention finds particular utility in the aluminum metal industry.
Briefly stated, an apparatus according to the present invention includes a bottom block or starter block for forming the butt end of a slab ingot in a semicontinuous casting station. The bottom block has a generally rectangular shape in plan view. When taken in a cross-sectional, longitudinal side view, the bottom block has a raised central region which tapers downwardly at opposed, transversely spaced ends thereof to form downwardly extending, depressed regions at opposed transverse ends. The raised central region and the transversely spaced depressed end regions of the bottom block are tapered at opposed side portions when viewed in a narrow edge side elevation to provide planar surfaces which intersect along a common line extending longitudinally along the long dimension of the block. In place of flat, planar surfaces forming the tapers, the tapers also may be formed by curved surfaces.
After ingot casting, the above-described specially shaped bottom block imparts a substantially like special shape to the butt end of the cast slab ingot. Those skilled in the art will appreciate that the solidified metal will shrink and curl away from the mold and assume a slightly dimensionally different shape. More specifically, if the bottom block shape is considered as the negative, the butt end of the ingot cast therein may be considered as the positive image thereof. Thus, the butt end of the ingot has longitudinally outwardly extending, enlarged portions which slope downwardly to a depressed central valley region. The lateral sides of the enlarged end portions and the depressed valley region carry tapered or curved edges. In addition, a similarly shaped hot top type mold may be employed to form the same or similar special shape at the head end of the ingot. At the conclusion of the casting run, when employing this embodiment, the molten metal is allowed to fill the specially shaped top mold to provide a slab ingot having a head end with a shape the same as, or similar to, the butt end. In this manner, cropping losses due to tongue and overlap problems are minimized at both ends of the rolled slab.
The present invention also contemplates the forming of the above-described special end shape to one or both ends of a conventionally cast slab ingot by machining or forging or like metal deformation technique after casting. While the machining or forging operation represents an additional cost element over in situ casting, it is believed that it will be more than offset by the savings realized through increased material recovery due to reduced end crop losses and increased rolling mill efficiency.
A process of the present invention includes the step of providing a slab ingot having at least one shaped end, preferably the butt end. The shaped end(s) has at least two longitudinally outwardly extending enlarged end portions at opposed, transversely spaced-apart locations adjacent to opposed edge or gage faces of the slab ingot having a region or regions of reduced longitudinal dimension or depressed valley therebetween. The shaped end(s) of the ingot also include upper and lower tapers transversely extending across the width of the slab into, respectively, from an upper rolling face and a lower rolling face of the ingot across the outwardly extending enlarged end portion and also across the depressed valley region of reduced longitudinal dimensions. The specially shaped end portions of the slab ingot are preferably formed by casting using a like-shaped end block and hot top mold. Alternatively, the specially shaped end may be formed by machining or forging a conventionally cast slab ingot. The presently preferred method of the invention, however, includes the step of providing a slab ingot with the special shape formed during casting by way of a shaped bottom block. The head end of the slab ingot may be left flat, as in conventional casting practice, or it may be subjected to a forming step through the use of a shaped mold, similar to a hot top mold, to form the above-described special shape at the head of the ingot at the conclusion of the casting run. In addition, the present invention contemplates that the head end, if left flat after casting, may be machined or forged to approximate the special shape of the butt end, including the enlarged end portions with the depressed intermediate valley therebetween, as well as the tapers transversely extending from the upper and lower rolling faces of the ingot. Alternatively, the head end and/or the butt end of the slab ingot, if left flat after casting, may be machined or forged or rolled partially only to provide transverse tapers across the upper and lower rolling faces (without the enlarged end portions) so as to minimize the overlapping problem at the head end of the rolled ingot.
A process of the present invention may also include the step of conducting an intermediate end cropping of the partially rolled slab in which the crop shear imparts a special shape to the slab. The cropped end includes enlarged end portions and a depressed central valley portion so as to minimize the formation of a tongue during subsequent rolling.
The process according to the present invention concludes by hot reverse rolling the slab ingot in a plurality of reducing passes through a hot reversing breakdown mill to reduce the thickness and increase the length of the ingot whereby the specially shaped slab ingot end(s) minimizes the formation of overlap and tongue so as to improve material recovery by reducing end crop losses and to increase rolling mill efficiency by increasing metal throughput.
These, as well as other advantages and features of the present invention, will become more readily apparent when reference is made to the appended drawings when taken in conjunction with the following detailed description.